1. Field of the Invention
The present invention generally relates to systems and methods for exhausting vapor and liquid. More particularly, the present invention relates to an exhaust system and method for a process bowl in a spin coating apparatus for the coating of wafer shaped semiconductor material.
2. Description of the Invention Background
Integrated circuits are typically constructed by depositing a series of individual layers of predetermined materials on a wafer shaped semiconductor substrate, or "wafer". The individual layers of the integrated circuit are in turn produced by a series of manufacturing steps. For example, in forming an individual circuit layer on a wafer containing a previously formed circuit layer, an oxide, such as silicon dioxide, is deposited over the previously formed circuit-layer to provide an insulating layer for the circuit. A pattern for the next circuit-layer is then formed on the wafer using a radiation alterable material, known as photoresist. Photoresist materials are generally composed of a mixture of organic resins, sensitizers and solvents. Sensitizers are compounds, such as diazonapthaquinones, that undergo a chemical change upon exposure to radiant energy, such as visible and ultraviolet light resulting in an irradiated material having differing solvation characteristics with respect to various solvents than the nonirradiated material. Resins are used to provide mechanical strength to the photoresist and the solvents serve to lower the viscosity of the photoresist so that it can be uniformly applied to the surface of the wafers. After a photoresist layer is applied to the wafer surface, the solvents are evaporated and the photoresist layer is hardened, usually by heat treating the wafer. The photoresist layer is then selectively irradiated by placing a radiation opaque mask containing a transparent portion defining the pattern for the next circuit layer over the photoresist layer and then exposing the photoresist layer to radiation. The photoresist layer is then exposed to a chemical, known as developer, in which either the irradiated or the nonirradiated photoresist is soluble and the photoresist is removed in the pattern defined by the mask, selectively exposing portions of the underlying insulating layer. The exposed portions of the insulating layer are then selectively removed using an etchant to expose corresponding sections of the underlying circuit layer. The photoresist must be resistant to the etchant, so as to limit the attack of the etchant to only the exposed portions of the insulating layer. Alternatively, the exposed underlying layer(s) may be implanted with ions which do not penetrate the photoresist layer thereby selectively penetrating only those portions of the underlying layer not covered by the photoresist. The remaining photoresist is then stripped using either a solvent, or a strong oxidizer in the form of a liquid or a gas in the plasma state. The next layer is then deposited and the process is repeated until fabrication of the semiconductor device is complete.
Photoresist and developer materials are typically applied to the wafer using a spin coating technique in which the photoresist is dispensed on the surface of the wafer as the wafer is spun on a rotating chuck. The spinning of the wafer distributes the photoresist over the surface of the material and exerts a shearing force that separates the excess photoresist from the wafer thereby providing a thin layer of photoresist on the surface of the wafer. Spin coating operations are performed in a clean room environment, and it is necessary to contain not only the excess coating material that is separated from the wafer, but also the vapor resulting from the evaporation of the solvent. In addition, photoresist materials are generally very expensive, ranging from $500 to $2300/gallon, therefore, reducing the amount of coating material used in the process can significantly reduce the overall cost of producing semiconductor devices. Also, a build up of excess coating material in the bowl requires additional downtime to remove and clean the bowl that further increases production costs.
FIG. 1 shows a side view of a typical bowl 200 and a porous wafer support chuck 202 of the prior art, such as is disclosed in U.S. Pat. No. 5,289,222 issued Feb. 22, 1994 to Hurtig. The wafer support chuck 202 is supported by a shaft 204 that passes through a hole 206 in the bowl 200 and attaches to a spin motor 208 in a motor compartment 209. A wafer 210 having a top and a bottom surface, 212 and 214 respectively, is placed on the wafer support chuck 202 and is secured using a vacuum (not shown). The wafer 210 is spun and coating material, such as photoresist or developer, is dispensed onto the top surface 212 of the wafer 210. The rotation of the wafer 210 causes the coating material to distribute over the top surface 212 and exerts a shear force on the coating material that separates excess coating material from the surface 212.
Some of the solvent in the excess coating material vaporizes upon leaving the surface producing dry aerosol particles of the coating mixed with the liquid drops which accumulate over time on wall 216 of the bowl 200. Also, the excess coating material has a tendency to creep around the edge of the wafer 210 and contaminate the bottom surface 214. If the coating material on the bottom surface 214 migrates to the chuck 202 a loss of vacuum could occur and the wafer 210 will be released, possibly damaging the wafer. A solvent spray nozzle 218 is attached to the bowl 200 and is directed toward the edge of the wafer 210 to rinse the bottom surface 214, thereby preventing a buildup of coating material. Solvent spray holes (not shown) are also provided in the bottom 217 of the bowl 200 to rinse the coating solution from the bottom surface.
The excess liquid coating and liquid solvent are drained from the bowl 200 using drain line 220 and the solvent vapors are purged from the bowl 200 with air through air purge line 222. Solvent vapors are exhausted from the motor compartment 209 through a safety exhaust line 224. The drain line 220, the air purge line 222 and the safety exhaust line 224 are connected to an exhaust manifold and the vapor and liquid are separated and either reclaimed or disposed accordingly.
One problem that exists with the prior art design shown in FIG. 1 is that in the region between the bottom surface 214 of the wafer 210 and the bottom of the bowl 217 a low pressure zone is created that results in a recirculation zone being formed that increases the amount of contamination that reaches the bottom surface 214 of the wafer 210, the bottom of the bowl 217, the chuck 202, and the motor 208. The recirculation zone results in a lower production yield due to contamination of the wafers and an increase in the overall processing time due to the increased downtime required to clean the bowl 200.
One prior art effort to eliminate the recirculation zone, shown in FIG. 2, employs a bowl 200 having a bottom 217 that is in close proximity to the bottom surface 214. While this design does eliminate the recirculation zone beneath the bottom surface 214, the pressure differential between the edge of the wafer and the axis of rotation and the proximity of the bottom 217 to the bottom surface 214 produces a wicking effect that draws coating material in toward the center of the bowl 200. The proximity of the bottom surface 214 to the bottom 217 of the bowl 200 also makes it more difficult to rinse the coating material off the bottom surface 214 using the solvent spray nozzle 218.
Another problem is that prior art bowls are generally segregated, such as by divider 226, to prevent the excess coating material from getting splashed or drawn onto the bottom surface 214 of the wafer 210. While this design is effective for that purpose, the solvent is also segregated from the excess coating material that is removed from the wafer 210 and the dry aerosol particulates that are produced as the solvent in the coating evaporates, all of which makes it more difficult to remove the liquid and solid coating material from the bowl 200. The problems of the liquid coating drying and forming a build up occurs not only in the bowl, but in the drain lines leading to the exhaust manifold, which, of course, leads to increased downtime to clean the bowl and the drain lines. The amount of downtime required to clean the bowl in the prior art is further increased by the fact that in order to fully clean the bowl or the chuck and motor or to perform maintenance, the bowl and chuck have to be disassembled to separate the components. Thus, it is apparent that a need exists for an improved spin coating bowl design which overcomes, among others, the above-discussed problems so as to provide a spin coating bowl that requires less maintenance and the maintenance that is performed requires less overall downtime.